The invention relates to a semiconductor device with a high-voltage lateral MOS transistor (HV-LDMOST), comprising a semiconductor body with a comparatively weakly doped silicon substrate of a first conductivity type and provided thereon a comparatively weakly doped epitaxial layer adjoining a surface of the semiconductor body, while the transistor comprises a comparatively strongly doped drain zone situated at the surface and of a second conductivity type opposed to the first conductivity type, a comparatively weakly doped drift region of the second conductivity type, a comparatively strongly doped source zone of the second conductivity type which adjoins the surface and is provided in a base region of the first conductivity type also adjoining the surface, and a control electrode provided above the surface and electrically insulated therefrom.
Such a device is known, for example, from the published Patent Application EP-A 0 514 060 A2. The epitaxial layer in this known device is of the same conductivity type as the substrate, so the p-type in usual embodiments. The drift region is formed by a comparatively weakly doped n-type surface layer provided in the epitaxial layer. The base region or "body region" of the transistor is formed by a p-type surface zone which is provided in the epitaxial layer and which is usually short-circuited with the strongly doped n-type source zone formed in this zone. Below the base region and conductively connected thereto, a strongly doped p-type buried layer is provided at the boundary between the high-ohmic p-type substrate and the also high-ohmic epitaxial layer. Peaks in the electric field distribution at the edge of the control electrode above the drift region are smoothed out by this buried zone, so that the breakdown voltage can be increased. Depending on various parameters such as the doping concentrations and thicknesses of various regions, a breakdown voltage of a few hundreds of volts up to more than 1,000 volts may be obtained. The RESURF principle known from the literature may advantageously be used for increasing the breakdown voltage, while the doping and thickness of the drift region are so chosen that this region is depleted throughout its thickness at least locally in a direction transverse to the surface, for example from the pn-junction opposite the surface, possibly in conjunction with field plates at the surface, before breakdown has occurred. This depletion leads to a reduction in the electric field and thus an increase in the breakdown voltage.
In many applications in which strong currents are switched on and off, high voltage peaks occur at the drain of the switching transistor upon switching-off, in particular as a result of inductive loads. In a vertical DMOST, breakdown across the pn-junction between the back gate and the drain may be use the inductance. This breakdown may be brought to a desired value by means of an extra doping in the back gate outside the channel region. Such a provision, however, is not known in the present art of a lateral high-voltage DMOST.
In versions of a lateral high-voltage DMOST in which the length of the drift region is sufficiently great, breakdown across the pn-junction of the drain zone opposite the surface will in the end still take place with an increasing voltage at the drain, so that the inductively stored power can be drained off through the source contact. This solution, however, is not feasible in most cases because usually a smaller distance between the source and drain zones is chosen with the object of keeping the on-resistance (R.sub.on) of the transistor low. As a result, the breakdown voltage of the pn-junction between the drain zone and the semiconductor body lies at a much higher level than the BV.sub.ds (source-drain breakdown voltage) of the transistor. This latter form of breakdown often has the result that certain properties of the transistor such as, for example, the threshold voltage change, sometimes even leads to destruction of the transistor, and is accordingly not useful for dealing with said voltage peaks at the drain.